Boric Acid Self-Lubrication of B2O3-Filled Polymer Composites

Utility of boric oxide particles in PTFE and epoxy composite materials, in sliding contact with stainless steel, is explored. Boric oxide filler can provide PTFE with a two-decade reduction in wear rate, to 10^?5 mm₃/N-m. With adequate ambient humidity reduced wear rate can be achieved without inducing counterface abrasion, and the friction of PTFE is further reduced slightly. In such environments, boric oxide fillers can also reduce friction coefficient of epoxy from μ>0.7 to as low as μ=0.07. This lubrication mechanism results from replenishment of lubricous boric acid lamellar solid provided to the sliding interface by reaction of boric oxide with ambient water. Maintenance of the lubricating effect depends upon a sufficient rate of boric acid formation, relative to subsequent removal by wear. It is demonstrated that this formation/removal balance is affected by relative humidity and volume fraction of boric oxide filler, as well as normal load and sliding speed.     

Transfer Solid Lubrication of Aluminum Sliding Contacts

The effects of transfer from solid lubricant sticks of unfilled, glass-filled, and bronze-filled PTFE on the room-temperature wear and friction of trailing primary contacts of aluminum (6061 T6) rods in repetitive intermittent contacts were investigated in a ring-on-rod configuration. The materials of the ring countersurfaces upon which the solid lubricants transferred and against which the trailing aluminum rods wore included steel, aluminum, copper, and an oxide dispersion-strengthened copper alloy. This sliding of the unlubricated copper ring countersurfaces against the aluminum led to the roughening of the copper as large (> 1 mm) aluminum particles embedded themselves upon the countersurface, with consequent transitions in the aluminum wear rate and the coefficient of friction to values exceeding 6 × 10^{? 3} mm³/Nm and 0.6, respectively, after an incubation period of several initial contacts of lower wear rate and friction. The other ring countersurface materials resulted in similarly high aluminum rod wear rate and coefficient of friction, more nearly from the onset of sliding. The application of unfilled PTFE solid lubricant transfer reduced the aluminum's gouging of the copper countersurfaces and correspondingly reduced the aluminum rod wear rate and the coefficient of friction against the copper, as well as against all other countersurface materials, towards 2 × 10^?3 mm³/Nm and 0.3 or less, respectively. Glass- and bronze-filled PTFE transfer lubricants provided reductions in the wear rate of the aluminum rod comparable to or in some cases better than the unfilled PTFE, though the unfilled PTFE transfer lubricant in several cases provided better friction reduction.     

Wear resistant solid lubricant coating made from PTFE and epoxy

A composite coating of polytetrafluoroethylene and epoxy shows 100 × improvements in wear resistance as compared to either of its constituents alone and reduced friction coefficient under testing on a pin-on-disk tribometer. This coating is made by impregnating an expanded PTFE film with epoxy, which provides three unique functions: (1) the epoxy compartmentalizes the PTFE nodes, which is believed to reduce the wear of the PTFE, (2) the epoxy increases the mechanical properties such as elastic modulus and hardness, and (3) the epoxy provides a ready interface to bond the films onto a wide variety of substrates easily and securely. The experimental matrix had normal loads of 1–3 N, sliding speeds from 0.25 to 2.5 m/s, and used a 2.4 mm radius low carbon steel pin in a rotating pin-on-disk tribometer. The skived PTFE films had wear rates on the order of K=10^–3 mm³/Nm and friction coefficients around =0.2. Both the high density films (70 wt%PTFE) and low density films (50 wt% PTFE) had wear rates on the order of K=10^–6 mm³/Nm and friction coefficients around =0.15. The neat epoxy films showed significant scatter in the tribological measurements with wear-rates on the order of K=10^–4 mm³/Nm and friction coefficients around =0.40. The enhanced tribological behavior of these composites is believed to stem from the coatings ability to draw thin PTFE transfer films into the contact from the nodes of PTFE, which act like reservoirs. Nanoindentation mapping of the coatings and the transfer films supports this hypothesis, and accompanies scanning electron microscopy observations of the worn and unworn coatings.     

Low Wear Steel Counterface Texture Design: A Case Study Using Micro-pits Texture and Alumina–PTFE Nanocomposite

Laser-induced surface micro-pits pattern has been successfully used under fluid lubrication to reduce friction and wear through mechanisms of enhanced hydrodynamic lubrication and fluid retention. Limited successes of friction and wear reduction using solid lubricant and textured surfaces have been reported in the literature, and there still lacks an efficient way of finding textures that produce desired tribological performances. This study evaluates the effect of counterface micro-pits texture on wear of a notable alumina–PTFE nanocomposite and uses the Taguchi method and “Simplex Method” to find the micro-pits parameters producing the lowest wear of the composite material. The optimum texture found yields a composite wear rate of 1 × 10^?7 mm³/Nm, a value identical to the material’s wear rate against untextured counterface. However, when slid against a freshly replaced composite pin, the existing transfer film on the optimum texture reduces composite’s wear volume at low wear transition by 90% and yields a steady-state wear rate of 3.9 × 10^?7 mm³/Nm. On the contrary, preexisting low wear transfer film on untextured counterface increases wear of the newly replaced pin by 10× and yields a wear rate of 4.4 × 10^?6 mm³/Nm. Results in this study suggest larger, shallower and sparser counterface pits are more favorable for debris entrapment, transfer film formation and wear reduction when slid against polymeric solid lubricants. It also raises new possibilities of self-adapting low wear counterface texture design that could potentially support low wear without requiring large amounts of run-in wear volume of bulk solid lubricants.     

Effect of Graphene and Ionic Liquid Additives on the Tribological Performance of Epoxy Resin

In the present study, we have determined the effect of graphene (PG), the ionic liquid (IL) and PG modified by mechanical blend with the IL 1-octyl-3-methylimidazolium tetrafluorobotate (IL + PG) on the tribological performance of epoxy resin (ER). IL + PG stable suspensions have been added to an epoxy resin (ER) matrix to obtain the new nanocomposite (ER + IL + PG), and its tribological performance has been compared with that of neat epoxy resin and with the nanocomposites containing PG (ER + PG) or IL (ER + IL). While neat ER presents a high dynamic friction coefficient of up to 0.31 and a severe wear with a specific wear rate of 8.1 × 10^?4 mm³ N^?1m^?1, the new nanocomposites show negligible surface damage, as determined by surface roughness and profilometry. All nanocomposites show low friction coefficients and negligible wear. The maximum friction reduction, up to a 70 %, is obtained for ER + PG. Results are discussed upon the basis of TEM microscopy, SEM microscopy and EDX analysis, differential scanning calorimetry, thermogravimetric analysis and dynamic mechanical analysis. Addition of IL or IL + PG has a plasticizing effect on ER, while addition of PG increases the thermal stability and stiffness of ER. PG shifts the storage modulus onset and in the loss modulus and tan δ maximum peaks to higher temperatures, while a shift to lower values is observed with addition of IL or IL + PG.     

Experimental Evaluation of Friction and Wear Properties of Solid Lubricant Coatings on SUS440C Steel in Liquid Nitrogen

The friction and wear properties of the prevailing different solid lubricant coatings (Ion-plated Au, Ion-plated Ag and RF-sputtered PTFE on SUS440C stainless steel) used in the bearings of high-speed cryogenic-turbo-pumps of liquid rocket engines were experimentally evaluated in liquid nitrogen immersed conditions. Also the above experiments were carried out with two newly proposed solid lubricant coatings of sputter-ion-plated MoSTi and a new ion-plated Pb on SUS440C stainless steel. The friction coefficient and wear rates of the coatings of ion-plated Au, ion-plated Ag, RF-sputtered PTFE, the new ion-plated Pb and MoS₂Ti-SIP (with coating thickness of 0.7±0.1 μm) on SUS440C steel against SUS440C stainless steel ball in liquid nitrogen were compared. Worn surfaces were examined microscopically with a microscope and a profilometer for understanding the mechanisms of friction and wear and transfer film lubrication in liquid nitrogen. It is found that the newly proposed solid lubricant coatings are showing promising results for their use in liquid nitrogen immersed conditions. The sputter-ion-plated MoSTi coating on SUS 440C steel shows a minimum value of friction coefficient (μ=0.015) and wear rate (w_c=0.56 × 10⁻⁶ mm³/N m ) in liquid nitrogen.     

Improved wear resistance in alumina-PTFE nanocomposites with irregular shaped nanoparticles

Past studies with PTFE nanocomposites showed up to 600× improvements in wear resistance over unfilled PTFE with the addition of Al₂O₃ nanoparticles. Irregular shaped nanoparticles are used in this study to increase the mechanical entanglement of PTFE fibrils with the filler. The tribological properties of 1, 2, 5 and 10 wt.% filled samples are evaluated under a normal pressure and sliding speed of 6.3 MPa and 50.8 mm/s, respectively. The wear resistance was found to improve 3000× over unfilled PTFE with the addition of 1 wt.% nanoparticles. The 5 wt.% sample had the lowest steady state wear rate of K = 1.3 × 10⁻⁷ mm³/N m and the lowest steady friction coefficient with μ = 0.21.     

Tribo-investigation on PTFE lubricated PEEK in harsh operating conditions

A series of blends with Polytetrafluroethylene (PTFE) powder and Polyetheretherketone (PEEK) was developed by varying the PTFE contents in steps of 5 wt.% from 0 to 20 wt.%. The composites were evaluated for their friction and wear properties at room temperature as well as high temperature in low amplitude oscillating wear (LAOW) mode against steel (100 Cr 6) ball against polymer plate. The same blends were also evaluated in abrasive wear mode to study the influence of harsh operating conditions on wear and friction performance. Incorporation of PTFE benefited PEEK in various ways such as it increased the tribo-utility of the latter by increasing its limiting load value, removing its stick-slip tendency, lowering coefficient of friction and specific wear rate significantly. With increase in PTFE content, benefits to the wear performance increased regularly. This was not the case for friction coefficient. Lowest μ was recorded for 15% PTFE contents. The enhancement in wear and friction performance, however, was at the cost of strength properties which decreased substantially with increase in PTFE content. At 100 °C, friction coefficient and wear rates of all blends increased marginally. In abrasive wear mode, on the other hand, PTFE filled PEEK showed poorer wear resistance than neat PEEK. This was correlated with strength properties and it was observed that these blends closely followed the predictions of Ratner–Lancaster plot. SEM was used to examine the micro-structural features of worn surfaces.     

Tribological behaviors and mechanisms of Ti<Subscript>3</Subscript>AlC<Subscript>2</Subscript>

Tribological behaviors and the relevant mechanism of a highly pure polycrystalline bulk Ti₃AlC₂ sliding dryly against a low carbon steel disk were investigated. The tribological tests were carried out using a block-on-disk type high-speed friction tester, at the sliding speeds of 20–60 m/s under a normal pressure of 0.8 MPa. The results showed that the friction coefficient is as low as 0.1∼0.14 and the wear rate of Ti₃AlC₂ is only (2.3–2.5) × 10⁻⁶ mm³/Nm in the sliding speed range of 20–60 m/s. Such unusual friction and wear properties were confirmed to be dependant dominantly upon the presence of a frictional oxide film consisting of amorphous Ti, Al, and Fe oxides on the friction surfaces. The oxide film is in a fused state during the sliding friction at a fused temperature of 238–324 °C, so it takes a significant self-lubricating effect.     

In Vacuo Tribological Behavior of Polytetrafluoroethylene (PTFE) and Alumina Nanocomposites: The Importance of Water for Ultralow Wear

Polytetrafluoroethylene (PTFE) is widely regarded as an excellent candidate for solid lubrication in vacuum. However, it is often precluded from many practical applications due to its intrinsically high wear rate. Over the past decade, it has been discovered that small loading fractions of alumina nanofillers can increase the wear resistance of PTFE by three to four orders of magnitude. This dramatic increase in wear resistance has in turn prompted numerous tribological studies to examine the robustness of this performance. In this study, the wear and friction behavior of unfilled PTFE and PTFE and alumina nanocomposites were evaluated under a broad range of vacuum environments from 760 to 4 × 10^?6 Torr. The nanocomposites of PTFE/alumina showed a dramatic increase in wear of over two orders of magnitude at the highest vacuum conditions. There appears to be an optimal vacuum environment around 1–10 Torr, in which these samples achieved the lowest wear rates of approximately 2.5 × 10^?7 mm³/(Nm).